Polyaxial Bone Fixation Element

ABSTRACT

The present disclosure includes a polyaxial bone fixation element for use in spinal fixation to interconnect a longitudinal spinal rod with a patient&#39;s vertebra. The polyaxial bone fixation element preferably includes a bone anchor, a collet, a body, and a locking cap. The polyaxial bone fixation element preferably enables in-situ assembly. That is, the polyaxial bone fixation element is preferably configured so that in use, the bone anchor may be secured to the patient&#39;s vertebra prior to being received within the body. Accordingly, the polyaxial bone fixation element enables a surgeon to implant the bone anchor without the body to maximize visibility and access around the anchoring site. Once the bone anchor has been secured to the patient&#39;s vertebra, the body can be snapped-onto the bone anchor. The bone anchor preferably also includes a second tool interface so that a surgical instrument can be directly coupled to the bone anchor.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/172,995 filed Oct. 29, 2018, which is a continuation of U.S.application Ser. No. 15/254,382 filed Sep. 1, 2016, now U.S. patent Ser.No. 10/136,923 issued Nov. 27, 2018, which is a continuation of U.S.application Ser. No. 14/163,482 filed Jan. 24, 2014, now U.S. Pat. No.9,439,681 issued Sep. 13, 2016, which is a continuation of U.S.application Ser. No. 12/669,224 filed Jan. 15, 2010, now U.S. Pat. No.8,663,298 issued Mar. 4, 2014, which is a national phase under 35 U.S.C.§ 371 of PCT Application No. PCT/US2008/070670 filed Jul. 21, 2008,which claims the benefit of U.S. Provisional Application No. 60/950,995filed Jul. 20, 2007, and U.S. Provisional Application No. 60/988,584filed Nov. 16, 2007, the entire contents of which are incorporatedherein by reference in their entireties.

BACKGROUND

It is often necessary due to various spinal disorders to surgicallycorrect and stabilize spinal curvatures, or to facilitate spinal fusion.Numerous systems for treating spinal disorders have been disclosed.

One method involves a pair of elongated members, typically spinal rods,longitudinally placed on the posterior spine on either side of spinousprocesses of the vertebral column. Each rod is attached to variousvertebrae along the length of the spine by way of pedicle screws. Thepedicle screws each may include a body having a U-shaped rod-receivingchannel for receiving a portion of the longitudinal spinal rod therein.Moreover, the body often interacts with a locking cap to clamp andsecure the position of the spinal rod within the rod-receiving channel.

To facilitate insertion of the spinal rod into the rod-receivingchannels and to provide additional flexibility in the positioning of thespinal rods and the pedicle screws, pedicle screws have been developedwherein the body is pivotable with respect to the bone anchor (commonlyknown as polyaxial pedicle screws).

It is desirable to develop a pedicle screw that is simple for a surgeonto use, provides for polyaxial rotation and is able to securely mountthe rod to the selected vertebra.

SUMMARY

A preferred embodiment of the present invention is directed to apolyaxial bone fixation element for use in a spinal fixation procedure.The polyaxial bone fixation element preferably includes a bone anchorhaving an enlarged head portion (e.g., a bone screw), a collet (e.g., aninsert member), a body having an axial bore for receiving the collet andthe enlarged head portion of the bone anchor. The body also includes arod-receiving channel and threads for threadably receiving a locking cap(e.g., an externally threaded set screw). The polyaxial bone fixationelement preferably enables in-situ assembly. That is, the polyaxial bonefixation element is preferably configured so that in use, the boneanchor may be secured to the patient's vertebra prior to being receivedwithin the body. Accordingly, the polyaxial bone fixation elementpreferably enables a surgeon to implant the bone anchor without the bodyand collet to maximize visibility and access around the anchoring site.Once the bone anchor has been secured to the patient's vertebra, thebody can “pop-on” to the bone anchor. The bone anchor may also includean instrument interface so that a surgical instrument can be directlycoupled to the bone anchor.

In one preferred embodiment, the polyaxial bone fixation elementincludes a bone anchor, a body, a collet and a locking cap. The boneanchor preferably includes an enlarged head portion. The head portionpreferably includes a drive surface for engaging a first surgicalinstrument and an instrument interface for engaging a second surgicalinstrument. The body preferably includes a longitudinal axis, an upperend with an upper opening, a lower end with a lower opening, a boreextending between the upper opening and the lower opening wherein thebore has a first diameter, and a rod-receiving channel for receiving thespinal rod. The rod-receiving channel has a channel axis that isoriented substantially perpendicular to the longitudinal axis. The bodypreferably also includes a lower edge portion adjacent the loweropening. The lower edge portion has a second diameter smaller than thefirst diameter. The collet preferably includes a first end, a second endand one or more slots extending from the second end, wherein the slotsdefine a plurality of flexible arms. The collet is preferably movablypositioned within the bore of the body. The locking cap is preferablyremovably engageable with the body. The locking cap is movable from anunlocked position to a locked position, wherein movement of the lockingcap from the unlocked position to the locked position urges the rodagainst the collet and the flexible arms against the lower edge portionto secure a position of the bone anchor relative to the body.

In another preferred embodiment, the polyaxial bone fixation elementincludes a body sized and configured to snap onto a head portion of animplanted bone anchor. The body preferably includes a longitudinal axis,an upper end with an upper opening, a lower end with a lower opening, abore extending between the upper and lower openings wherein the bore hasa first diameter, and a rod-receiving channel extending from the upperend toward the lower end and positioned on a channel axis that isoriented substantially perpendicular to the longitudinal axis. The borepreferably includes a lower edge portion terminating proximate the lowerend and an enlarged diameter portion disposed adjacent to the lower edgeportion and between the lower edge portion and the upper end. The loweredge portion preferably has a second diameter while the enlargeddiameter portion has a third diameter, wherein the third diameter ispreferably larger than the first diameter, which is larger than thesecond diameter. The collet preferably includes a first end, a secondend and one or more slots extending from the second end, wherein theslots define a plurality of flexible arms. The flexible arms preferablyeach have a root end, a terminal end and a generally spherical, externalsurface proximate the terminal end. The flexible arms render the colletexpandable to accept the head of the bone anchor and compressible tosecure the head of the bone anchor relative to the collet. The flexiblearms are preferably positioned proximate the enlarged diameter portionin a loading position and at least a portion of the external surface ofthe flexible arms contact the lower edge portion in a locked position.

In an alternate preferred embodiment, the polyaxial bone fixationelement preferably includes a bone anchor, a body and a collet. The boneanchor preferably includes a head portion, wherein the head portionincludes a drive surface for engaging a first surgical instrument and aninstrument interface for engaging a second surgical instrument. The bodypreferably includes a longitudinal axis, an upper end with an upperopening, a lower end with a lower opening, a bore extending between theupper and lower openings, and a rod-receiving channel extending from theupper end toward the lower end and positioned on a channel axis that isoriented substantially perpendicular to the longitudinal axis. The borepreferably also includes a lower edge portion proximate the lower endand an enlarged diameter portion adjacent to the lower edge portion andbetween the lower edge portion and the upper end. The collet ispreferably movably positioned within the bore of the body. The colletpreferably includes a first end, a second end and one or more slotsextending from the second end, wherein the slots define a plurality offlexible arms. The flexible arms preferably render the collet expandableto accept the head of the bone anchor and compressible to secure thehead of the bone anchor relative to the collet. The collet preferablyfurther includes a cavity extending from the second end. The flexiblearms of the collet are preferably positioned in general verticalalignment with the enlarged diameter portion in a loading position sothat the head of the bone anchor can be received in the cavity formed inthe collet. At least a portion of the flexible arms preferably contactthe lower edge portion in a locked position so that the head of the boneanchor is secured with respect to the collet. In the locked position,the contact is generally a line contact between the collet and the body.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofa preferred embodiment of the application, will be better understoodwhen read in conjunction with the appended drawings. The preferredembodiment of the polyaxial bone fixation element is shown in thedrawings for the purposes of illustration. It should be understood,however, that the application is not limited to the precise arrangementsand instrumentalities shown. In the drawings:

FIG. 1 illustrates an exploded, perspective view of a preferredembodiment of a polyaxial bone fixation element;

FIG. 2A illustrates a front elevational view of a portion the polyaxialbone fixation element shown in FIG. 1, mounted in a patient's vertebra;

FIG. 2B illustrates a side elevational view of the portion of thepolyaxial bone fixation element shown in FIG. 1, mounted in thepatient's vertebra;

FIG. 3 illustrates a partial, cross-sectional view of the polyaxial bonefixation element shown in FIG. 1, taken along line 3-3 of FIG. 2B;

FIG. 4A illustrates a magnified cross-sectional view of a head portionof a bone anchor used in connection with the polyaxial bone fixationelement shown in FIG. 1;

FIG. 4B illustrates a top plan view of the bone anchor shown in FIG. 4A;

FIG. 5A illustrates a front elevational view of a collet used inconnection with the polyaxial bone fixation element shown in FIG. 1;

FIG. 5B illustrates a side elevational view of the collet shown in FIG.5A;

FIG. 5C illustrates a cross-sectional view of the collet shown in FIG.5A, taken along line 5C-5C of FIG. 5B;

FIG. 6A illustrates a front elevational view of a first preferredembodiment of a body used in connection with the polyaxial bone fixationelement shown in FIG. 1;

FIG. 6B illustrates a side elevational view of the body shown in FIG.6A;

FIG. 6C illustrates a cross-sectional view of the body shown in FIG. 6A,taken along line 6C-6C of FIG. 6B;

FIG. 6D illustrates a magnified, cross-sectional view of a lower end ofthe body shown in FIG. 6A, taken from within circle 6D of FIG. 6C;

FIG. 6E illustrates a magnified, cross-sectional view of the lower endof the body shown in FIG. 6D and a collet and head of a bone anchor ofthe polyaxial bone fixation element shown in FIG. 1;

FIG. 6F illustrates a magnified, cross-sectional view of a secondpreferred embodiment of a lower end of a body of the polyaxial bonefixation element shown in FIG. 1;

FIG. 6G illustrates a magnified, cross-sectional view of the lower endof the body shown in FIG. 6F;

FIG. 7A illustrates a top plan view of a locking cap used in connectionwith the polyaxial bone fixation element shown in FIG. 1;

FIG. 7B illustrates a cross-sectional view of the locking cap shown inFIG. 7A, taken along line 7B-7B of FIG. 7A;

FIG. 8A illustrates a side perspective view of a preferred embodiment ofa screw driver and a sleeve coupled to a portion of the polyaxial bonefixation element of FIG. 1;

FIG. 8B illustrates a cross-sectional view of the screw driver andsleeve coupled to the portion of the polyaxial bone fixation element ofFIG. 1, taken along line 8B-8B of FIG. 8A;

FIG. 9A illustrates a cross-sectional view of a syringe assembly and thesleeve coupled to a portion of the polyaxial bone fixation element ofFIG. 1; and

FIG. 9B illustrates a magnified, cross-sectional view of the syringeassembly and sleeve shown in FIG. 9A, taken from within the circle 9B ofFIG. 9A.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Certain terminology is used in the following description for convenienceonly and is not limiting. The words “right”, “left”, “lower” and “upper”designate directions in the drawings to which reference is made. Thewords “inwardly” and “outwardly” refer to directions toward and awayfrom, respectively, the geometric center of the polyaxial bone fixationelement, the described instruments and designated parts thereof. Thewords, “anterior”, “posterior”, “superior”, “inferior” and related wordsand/or phrases designate preferred positions and orientations in thehuman body to which reference is made and are not meant to be limiting.The terminology includes the above-listed words, derivatives thereof andwords of similar import.

Certain exemplary embodiments of the invention will now be describedwith reference to the drawings. In general, such embodiments relate to apreferred polyaxial bone fixation element, and related instruments byway of non-limiting example and a polyaxial bone fixation element foruse in spinal fixation to facilitate insertion of a longitudinal spinalrod in a rod-receiving channel formed in the body of the polyaxial bonefixation element. The invention may have other applications and uses andshould not be limited to the structure or use described and illustrated.

Referring to FIGS. 1-7B, a preferred polyaxial bone fixation element 10includes a bone anchor 20 (shown as a bone screw), a collet 150, a body200, and a locking cap 300 (shown as an externally threaded set screw).As will be described in greater detail below, the polyaxial bonefixation element 10 preferably enables in-situ assembly. That is,preferably, the polyaxial bone fixation element 10 is configured so thatin use, the bone anchor 20 may be secured to a patient's vertebra 700prior to being received within the body 200. The polyaxial bone fixationelement 10 preferably enables a surgeon to implant the bone anchor 20without the body 200 and collet 150 pre-assembled to the bone anchor 20.By enabling the surgeon to implant the bone anchor 20 only, thepolyaxial bone fixation element 10 maximizes visibility and accessaround the anchoring site. Once the bone anchor 20 has been secured tothe patient's vertebra 700, the body 200 and collet 150 may “pop-on” tothe bone anchor 20. Accordingly, the preferred polyaxial bone fixationelement 10 is typically considered a bottom loading device, because thebone anchor 20 enters the body 200 through a lower or bottom end 204.Alternatively, the polyaxial bone fixation element 10 may be providedpre-assembled using identical components as described herein or may beconfigured for top loading with minor modifications, as would beapparent to one having ordinary skill in the art. Further, the collet150 and body 200 assembly may be popped-off of the bone anchor 20in-situ by arranging the collet 150 relative to the body 200 in aloading position, after the fixation element 10 has been arranged in thelocked position, and removing the assembly from the bone anchor 20, aswill be described in greater detail below.

While the polyaxial bone fixation element 10 will be described as andmay generally be used in the spine (for example, in the lumbar, thoracicor cervical regions), those skilled in the art will appreciate that thepolyaxial bone fixation element 10 may be used for fixation of otherparts of the body such as, for example, joints, long bones or bones inthe hand, face, feet, extremities, cranium, etc.

As will be described in greater detail below, several polyaxial bonefixation elements 10 may be used to secure a longitudinal spinal rod 250to several vertebrae 700. It should be understood that the spinal rod250 may include, but is not limited to, a solid rod, a non-solid rod, aflexible or dynamic rod, etc. It should be understood that the polyaxialbone fixation element 10 is not limited in use to any particular type ofspinal rod 250.

Referring to FIGS. 1-4B, the bone anchor 20 preferably is in the form ofa bone screw 22. Alternatively, however, the bone anchor 20 may be, forexample, a hook or other fastener such as, a clamp, an implant, etc.

The bone screw 22 preferably includes an enlarged, curvate head portion24 and an externally threaded shaft portion 26 for engaging thepatient's vertebra 700. The specific features of the shaft 26 including,for example, thread pitch, shaft diameter, shaft shape, etc. areinterchangeable, and it would be apparent to one having ordinary skillin the art that the bone screw 22 is not limited to any particular typeof shaft 26. The bone screw 22 may or may not be cannulated (See FIGS.9A and 9B). The bone screw 22 may also include a reduced diameter neckportion 28 between the head portion 24 and the shaft portion 26, whichaccommodates the polyaxial nature of the bone fixation element 10. Thebone screw 22 may further be cannulated and fenestrated (not shown) suchthat openings extend outwardly from a central hollow channel in acannulated screw to urge fluid out of the screw during injection or drawfluid into the central hollow channel from sides of the screw duringextraction of material adjacent the screw.

Referring to FIGS. 3-4B and 8A-9B, the enlarged curvate head portion 24preferably has a curvate or semi-spherical shape to facilitate rotationwith respect to the collet 150, as will be described in greater detailbelow. The head portion 24 also preferably includes a drive surface 30for receiving a corresponding tip 501 formed on a drive tool, such as ascrew driver 500 (FIGS. 8A and 8B) for rotating the bone screw 22 intoengagement with the patient's vertebra 700. The drive surface 30 mayhave any form now or hereafter known including, but not limited to, anexternal hexagon, a star drive pattern, a Phillips head pattern, a slotfor a screw driver, a threading for a correspondingly threaded post,etc. Preferably, as shown, the drive surface 30 is comprised of a firsttool interface or an internal recess 32, but is not so limited and maybe comprised of an external drive feature that engages a female-typedriver (not shown). The specific shape of the drive surface 30 or firsttool interface 32 may be chosen to cooperate with the correspondingdrive tool.

The head portion 24 may also include a second tool interface or a sleeveinterface 40. The second tool interface 40 may be in any form now orhereafter known including, but not limited to, an internal or externalthread, an external hexagon, a star drive pattern, a Phillips headpattern, a slot for a screw driver, a groove, a slot, etc. Preferably,however, the second tool interface 40 includes a plurality of threads 42for threadably engaging a surgical instrument, such as a sleeve 600(FIGS. 8A-9B), bone augmentation instrumentation, aspirationinstrumentation, reduction tool for sagittal reduction or otherreduction, coronal rotation tool, soft tissue refraction tool, kyphosisand lordosis correction tool, etc. The second tool interface 40 of thepreferred embodiment permits application of forces to the bone anchor 20along a longitudinal axis of the bone anchor 20, as well as along or atangles to the axis. In the preferred embodiment, the sleeve 600 isadaptable for use in combination with at least the screw driver 500 andan injection assembly 650. The surgical instrument may alternatively beany surgical instrument now or hereafter used in connection with aspinal fixation procedure including, but not limited to, a compressor, adistractor, minimally invasive instrumentation, etc. By incorporatingthe second tool interface 40 into the head portion 24 of the bone anchor20, the sleeve 600 is able to directly engage the bone anchor 20 thuseliminating the need for the sleeve 600 to engage the body 200 of thepolyaxial bone fixation element 10 and thereby, limiting togglingbetween the sleeve 600, screw driver 500 and/or bone anchor 20 in aworking configuration, as will be described in greater detail below. Inaddition, the preferred second tool interface 40 permits application offorces to the bone anchor 20 through the sleeve 600 or another tool thatmates with the second tool interface 40 to manipulate the bone anchor 20and potentially the bone that the bone anchor 20 is mounted in.

Referring to FIGS. 3-4B, 8B and 9B, the second tool interface 40 and thefirst tool interface 32 are preferably formed in a head interface cavity41 exposed from the top end of the head 24. Exposing both the secondtool interface 40 and the first tool interface 32 at the top end of thehead 24 permits simultaneous engagement of instruments with the secondtool interface 40 and first tool interface 32 for manipulating the boneanchor 20. Both the second tool interface 40 and the first toolinterface 32 may be engaged individually or simultaneously by aninstrument prior to mounting the collet 150 and body 200 to the head 24or after the collet 150 and body 200 are mounted to the head 24 (SeeFIGS. 8B and 9B).

Referring to FIGS. 3 and 5A-5C, the collet 150 preferably includes afirst or upper end 152 sized and configured to contact at least aportion of the spinal rod 250 (schematically depicted in FIG. 5A) whenthe spinal rod 250 is received within a rod-receiving channel 208 formedin the body 200 and a second or lower end 154 sized and configured tocontact at least a portion of the head portion 24 of the bone anchor 20.More preferably, the upper end 152 of the collet 150 includes a seat 160sized and configured to receive at least a portion of the spinal rod 250when the spinal rod 250 is received within the rod-receiving channel 208of the body 200. The lower end 154 of the collet 150 preferably includesan interior cavity 165 for receiving and securing the head portion 24 ofthe bone anchor 20 so that, as will be generally appreciated by one ofordinary skill in the art, the bone anchor 20 can polyaxially rotatethrough a range of angles with respect to the collet 150 and hence withrespect to the body 200. The cavity 165 formed in the collet 150preferably has a curvate or semi-spherical shape for receiving thecurvate or semi-spherical head portion 24 of the bone anchor 20 so thatthe bone anchor 20 can polyaxially rotate with respect to the collet 150and hence with respect to the body 200. Furthermore, at least a portionof the outer surface of the collet 150 is comprised of a curvate orspherical, convex surface 151 having a radius of curvature r.sub.5 forcontacting the inner surface 211 of the body 200, preferably the loweredge portion 218, as will be described in greater detail below.

The collet 150 preferably also includes one or more slots 170 (shown asa plurality of slots) extending from the lower end 154 thereof so thatat least a portion of the collet 150 is: (i) radially expandable so thatthe head portion 24 of the bone anchor 20 can be inserted through thelower end 154 and into the cavity 165 of the collet 150 and (ii)radially compressible to compress or crush-lock against the head portion24 of the bone anchor 20, in accordance with the application of radialforces applied thereto. In the preferred embodiment, the slots 170define a plurality of flexible arms 172. Preferably each flexible arm172 includes a root end 173 and a terminal end 174. The outer surface ofthe flexible arms 172 preferably include the curvate or spherical convexsurface 151 of the collet 150 for defining a line-contact with the innersurface 211 of the body 200, preferably the first undercut 218 a, aswill be described in greater detail below.

The collet 150 may also include one or more grooves 175 formed on theouter surface thereof for engaging a projection or dimple 216 a formedin the inner surface 211 of the body 200. As will be described ingreater detail below, the collet 150 is permitted to float within theaxial bore 206 formed in the body 200 between a loading position and alocked position. That is, the collet 150 is preferably movablypositioned within the body 200 in an assembled configuration.Interaction between the one or more grooves 175 and the projection ordimples 216 a prevents the collet 150 from moving out of the upper end202 of the body 200 when in the loading position.

The collet 150 also includes a bore 156 extending from the upper end 152to the lower end 154 with an upper opening at the upper end 152 so that,for example, a drive tool, such as, for example, a screw driver 500, canbe inserted through the collet 150 and into engagement with the boneanchor 20 so that the bone anchor 20 may be rotated into engagement withthe patient's vertebra 700. The upper opening at the upper end 152 ofthe collet 150 also permits simultaneous insertion of the screw driver500 and a second tool, such as the sleeve 600, therethrough to engagethe head 24 (FIGS. 8B and 9B).

The collet 150 may also include one or more provisional rod-lockingfeatures so that the spinal rod 250 may be provisionally coupled to thecollet 150, and hence with respect to the body 200. The provisionalrod-locking features may be any mechanism now or hereafter developed forsuch purpose.

Referring to FIG. 5C, the collet 150 includes one or more inwardlyprojecting ledges 184, 186 disposed on an inner surface 161 of the seat160 adjacent the upper end 152 of the collet 150. The ledges 184, 186may be engaged by a tool (not shown) to apply a force between the collet150 and the body 200 to move the collet 150 relative to the body 200.For example, the body 200 may be urged downwardly toward the bone anchor20 relative to the collet 150 when the collet 150, body 200 and boneanchor 20 are in the locked position. Such application of a force maymove the collet 150 from the locked position into the loading position,in situ, such that the flexible arms 172 are able to flex outwardlywithin an enlarged diameter portion 220 to permit the head 24 to moveout of the cavity 165. Accordingly, the ledges 184, 186 may be utilizedto disassemble the collet 150 and body 200 from the bone anchor 20 afterthe collet 150 and body 200 have been locked to the head 24.

Referring to FIGS. 6A-6D and 7A-7B, the body 200 may generally bedescribed as a cylindrical tubular body having a longitudinal axis 201,an upper end 202 having an upper opening 203, a lower end 204 having alower opening 205, and an axial bore 206 substantially coaxial with thelongitudinal axis 201 of the body 200. The axial bore 206 extends fromthe upper opening 203 to the lower opening 205. The axial bore 206preferably has a first diameter portion D₁ proximate the upper end 202.The body 200 also includes a substantially transverse rod-receivingchannel 208 (shown as a top loading U-shaped rod-receiving channel)defining a pair of spaced apart arms 209, 210. The inner surface 211 ofthe spaced apart arms 209, 210 preferably includes a plurality ofthreads 212 for engaging a locking cap 300. Alternatively, the body 200and, in particular, the spaced apart arms 209, 210 may have nearly anymounting receiving structure for engaging the locking cap 300 including,but not limited to, external threads, cam-lock, quarter lock, clamps,etc. The outer surface 213 of the spaced apart arms 209, 210 may eachinclude a recess 214 for engaging one or more surgical instruments suchas, for example, rocker forceps, a compressor, a distractor, a sleeve,minimally invasive instrumentation, etc.

Referring to FIGS. 5A-6E, the axial bore 206 preferably has the firstdiameter portion D₁ proximate the upper end 202. The inner surface 211of the axial bore 206 preferably also includes a lower end portion 218proximate the lower end 204 thereof. The lower end portion 218 defines asecond diameter portion D₂, which is comprised of the smallest diameterportion of the axial bore 206. The second diameter portion D₂ ispreferably defined by a first spherical undercut 218 a adjacent thelower end 204 of the body 200. The first spherical undercut 218 apreferably has a second radius of curvature r2 that is centered on thelongitudinal axis 201 of the body 200. The second diameter portion D₂ ispreferably smaller than the first diameter portion D₁ of the axial bore206 such that the collet 150 may be inserted through the upper end 202into the axial bore 206, but generally preventing the collet 150 frombeing inserted into the lower end 204 or from falling out of the lowerend 204 once inserted into the axial bore 206.

The first spherical undercut 218 a is preferably defined as a curvate orspherical concave surface for accommodating the outer curvate orspherical convex surface 151 of the collar 150. The first sphericalundercut 218 a and the spherical convex surface 151 preferably have adifferent radius of curvature such that line contact is defined betweenthe surfaces 151, 218 when the collet 150 is positioned proximate thelower end 204. The second diameter portion D₂ is preferably sized andconfigured so that the enlarged head portion 24 of the bone anchor 20may be passed through the lower opening 205 of the body 200, but isprevented from passing therethrough once the head portion 24 of the boneanchor 20 is received within the interior cavity 165 of the collar 150.

The inner surface 211 of the axial bore 206 preferably includes anenlarged portion 220 that is located toward the lower end 204 relativeto the first diameter portion D₁. The enlarged portion 220 preferablydefines a third diameter D₃ comprised of a curvate, preferablyspherical, radially outwardly recessed portion. In the enlarged portion220 of the axial bore 206, the third diameter D₃ is larger than thefirst diameter D₁ of the axial bore 206. In addition, the third diameterD₃ is larger than the second diameter D₂. In the preferred embodiment,the third diameter D₃ is defined by a second spherical undercut 220 a.

The enlarged portion 220 is preferably located in between the upper end202 and the lower end portion 218 and accommodates expansion of theflexible arm 172 therein when the head 24 is loaded into the collet 150,as will be described in greater detail below. The enlarged portion 220is preferably in the form of a curvate or spherical concave surfacehaving a third radius of curvature r3, which defines the third diameterD₃ at the largest diameter within the axial bore 206. The third radiusof curvature r3 defines the spherical nature of the second sphericalundercut 220 a. The enlarged portion 220 is sized and configured so thatwhen the collet 150 is placed in general alignment with the curvate orspherical concave surface of the enlarged portion 220, the flexible arms172 of the collet 160 are permitted to radially expand within the axialbore 206 of the body 200 so that the head portion 24 of the bone anchor20 can be inserted through the lower opening 205 formed in the body 200and into the cavity 165 formed in the collet 150. More preferably, theenlarged portion 220 is sized and configured so that the outer curvateor spherical convex surface 151 of the collet 150 does not touch orcontact the enlarged portion 220 of the body 200 when the head 24 isloaded into the collet 150. That is, the enlarged portion 220 formed inthe body 200 is preferably sized and configured so that a gap remainsbetween the outer curvate or spherical convex surface 151 of the collet150 and the enlarged portion 220 of the body 200 even when the flexiblearms 172 radially expand to accept the head portion 24 of the boneanchor 20. The enlarged portion 220 is not limited to constructionscomprised of the preferred curvate or spherical undercut defined by thethird radius of curvature r3 and may be constructed of nearly anyundercut having nearly any shape that permits expansion of the collet150 therein in the loading position to accept the head 24. For example,the enlarged portion 220 may be defined by a rectangular slot or grooveon the inner surface 211 that results in the third diameter D₃ beinglarger than the first and second diameters D₁, D₂.

In the preferred embodiment, the second radius of curvature r2 of thefirst spherical undercut 218 a is preferably different than an outerradius of curvature r5 of the outer curvate or spherical convex surface151 of the collet 150 so a line contact results between the firstspherical undercut 218 a and the outer convex surface 151 when thecollet 150 is positioned adjacent the lower end portion 218. That is, byproviding non-matching radius of curvatures between the first sphericalundercut 218 a and the collet 150, only line contact occurs between thefirst spherical undercut 218 a of the body 200 and the outer curvate orspherical convex surface 151 of the collet 150. The line contact betweenthe body 200 and the collet 150 effectively pinches the lower ends ofthe flexible arms 172 onto the lower end of the head 24 below thegreatest diameter of the head 24 to direct the lower end 154 beneath thelargest diameter of the head 24, effectively locking the bone anchor 20to the collet 150 in the locked position. In addition, the line contactbetween the collet 150 and body 200 permits disengagement of the collet150 from the body 200 after the collet 150 and body 200 are engaged inthe locked position or popping-off of the body 200 and collet 150 fromthe bone anchor 20, in situ.

Referring to FIGS. 5A-5C, 6F and 6G, the second and third diameters D2,D3 may be formed by a single internal radius of curvature r4 thatundercuts the body 200 in the axial bore 206. The single internal radiusof curvature r4 preferably permits expansion of the collet 150 to acceptthe head 24, insertion of the collet 150 into the axial bore 206 fromthe upper end 202 but not the lower end 204 and line contact between theouter curvate or spherical convex surface 151 of the collet and thelower end portion 218 when the collet 150 is in facing engagement withthe lower end portion 218. In this configuration, the second diameterportion D2 is smaller than the first diameter portion D1, which issmaller than the third diameter portion D3.

Referring to FIGS. 1-3 and 5A-6C, the body 200 and collet 150 alsopreferably include a collet retention feature 50 so that once the collet150 has been inserted into the bore 206 formed in the body 200 and, ifnecessary, the collet retention feature 50 has been engaged, the colletretention feature 50 inhibits the collet 150 from passing back throughthe upper opening 203 formed in the body 200, but permits some degree ofvertical translation or floating of the collet 150 with respect to thebody 200. That is, once inserted into the axial bore 206 of the body200, the collet 150 is sized and configured to float or move within theaxial bore 206 between a loading position and a locking position. Thecollet retention feature 50 preferably prevents the collet 150 frommoving out of the upper opening 203 of the body 200. Preferably thecollet retention feature 50 permits the flexible arms 172 to align withthe enlarged portion 220 in the loading position when the lower edge ofthe grooves 175 come into contact with the dimples 216 a. In addition,the collet 150 is preferably permitted to float between the loadingposition and the locking position prior to locking of the head 24 in thecollet 150. Specifically, the collet 150 may float between the loadingposition where the dimples 216 a are in contact with the lower edge ofthe grooves 175 and the locking position wherein the outer curvate orspherical convex surface 151 is in line contact with the lower endportion 218. The collet retention feature 50 preferably limits rotationof the collet 150 with respect to the body 200, because the dimples 216a slide within the grooves 175, so that the seat 160 formed in thecollet 150 is aligned with the rod-receiving channel 208 formed in thebody 200. However, the retention feature 50 is not limited to limitingrotation of the collet 150 with respect to the body 200 and may beconfigured to permit unlimited rotation of the collet 150 relative tothe body 200 by eliminating the grooves 175 from the collet 150 andforming a shelf (not shown) around the collet 150 at the bottom end ofthe grooves 175 such that the dimples 216 a engage the shelf to limitremoval of the collet 150 out of the upper end 202 of the body 200, butpermit unlimited rotation of the collet 150 relative to the body 200 inthe assembled configuration.

The collet retention feature 50 may be any feature now or hereafterknown for such purpose including, but not limited to, for example, aninwardly protruding shoulder or detent formed on the collet 150 forengaging corresponding indentations formed on the inner surface 211 ofthe body 200. In the preferred embodiment, the body 200 includes one ormore partial passageways 216 formed therein so that once the collet 150has been received within the axial bore 206 of the body 200, a force maybe applied to the partial passageways 216 formed in the body 200deforming the remaining portion of the partial passageway 217 into thedimple or projection 216 a formed in the inner surface 211 of the body200. That is, once the collet 150 has been received within the bore 206of the body 200, an external force may be applied to the partialpassageways 216 formed in the body 200 transforming the passageways 216into the projections or dimples 216 a that extend inwardly from theinner surface 211 of the spaced apart arms 209, 210 and into the bore206 formed in the body 200. The dimples or projections 216 a arepreferably sized and configured to interact with the longitudinal groove175 formed in the outer surface of the collet 150 so that the collet 150is permitted to move with respect to the body 200 at least along thelongitudinal axis 201, but inhibited from moving back through the upperopening 203 formed in the body 200. The collet 150 is also preferablypartially inhibited from rotational movement with respect to the body200. Movement of the collet 150 with respect to the body 200 toward theupper end 202 is preferably inhibited by the projections or dimples 216a contacting the bottom and/or lateral surfaces of the grooves 175.Limiting rotational movement of the collet 150 with respect to the body200 permits alignment of the rod-receiving channel 208 and the seat 160for receiving the rod 250, as will be described in greater detail below.

In use, positioning the collet 150 in general alignment with the curvateor spherical concave surface of the enlarged portion 220 in the loadingposition preferably enables the flexible arms 172 of the collet 150 toradially expand within the axial bore 206 of the body 200 so that thehead portion 24 of the bone anchor 20 can be inserted through the loweropening 205 formed in the body 200 and into the cavity 165 formed in thecollet 150. The enlarged portion 220 formed in the body 200 ispreferably sized and configured so that a gap remains between the outercurvate or spherical convex surface 151 of the collet 150 and theenlarged portion 220 of the body 200 even when the flexible arms 172radially expand to accept the head portion 24 of the bone anchor 20.Thereafter, movement of the collet 150 into general alignment andengagement with the first spherical undercut 218 a of the lower endportion 218 causes a radial inward force to be applied to the flexiblearms 172, which in turn causes the flexible arms 172 to compress againstthe head portion 24 of the bone anchor 20, thereby securing the positionof the bone anchor 20 with respect to the collet 150 and hence withrespect to the body 200. The lower end portion 218 and the outer curvateor spherical convex surface 151 of the collet 150 have non-matchingradii of curvature r2, r4, r5 so that only line contact occurs betweenthese components.

The head portion 24 of the bone anchor 20 and interaction of the dimples216 with the grooves 175 preferably moves the collet 150 into alignmentwith the enlarged portion 220 as the head portion 24 is inserted throughthe lower opening 205 and into the axial bore 206. Moreover, the collet150 is preferably moved into alignment and engagement with the loweredge portion 218 via engagement of the locking cap 300, as will bedescribed in greater detail below.

Referring to FIGS. 7A and 7B, the locking cap 300 is preferably anexternally threaded set screw 302 for threadably engaging the threads212 formed on the inner surface 211 of the body 200. The externallythreaded set screw 302 generally provides flexibility when inserting aspinal rod 250 into the body 200 such that the spinal rod 250 does nothave to be completely reduced or seated within the body 200 prior toengagement of the cap 300. Incorporation of a threaded set screw 302also enables the set screw 302 to reduce the spinal rod 250 duringtightening of the locking cap with respect to the body 200. The lockingcap 300 may be any locking cap now or hereafter developed for suchpurpose including, but not limited to, an externally threaded cap, aquarter-turn or partial-turn locking cap, two-piece set screw, etc.

As shown, the externally threaded set screw 302 preferably includes adrive surface 304 for engaging a corresponding drive tool for securing(e.g., threading) the set screw 302 onto the body 200. The drive surface304 may take on any form now or hereafter developed for such purpose,including, but not limited to, an external hexagon, a star drivepattern, a Phillips head pattern, a slot for a screw driver, a threadingfor a correspondingly threaded post, etc. The drive surface 304 ispreferably comprised of an internal recess. The specific shape of theinternal recess may be chosen to cooperate with the corresponding drivetool. The drive surface 304 may also be configured to include the firstand second tool interfaces 40, as were described above.

The externally threaded set screw 302 preferably also includes a saddle310 operatively coupled thereto. The saddle 310 includes a transverserecess 312 formed therein for contacting at least a portion of thespinal rod 250. The rod-contacting surface of the recess 312 may includea surface finish (not shown) that adds roughness, such as, for example,a knurl, bead blasting, grooves, or other textured finish that increasessurface roughness and enhances rod push through strength.

The saddle 310 may be coupled to the set screw 302 by any means now orhereafter developed for such purpose including, but not limited to,adhesion, mechanically fastening, etc. The set screw 302 preferablyincludes a bore 306 for receiving a stem 316 formed on a top surface 311of the saddle 310. In use, the saddle 310 is preferably coupled to theset screw 302 but is free to rotate with respect to the set screw 302 sothat the saddle 310 can self-align with the spinal rod 250 while the setscrew 302 is being rotated with respect to the body 200.

In one particularly preferred embodiment, the threads formed on theexternally threaded set screw 302 may incorporate inclined load flanksforming an angle with respect to the longitudinal axis 201 of the body200. The load flanks may be converging so that the top surface of thethread and the bottom surface of the thread converge. The angle may beabout five degrees (5.degree.), although, as will be generallyappreciated by one of ordinary skill in the art, the threads may take onany other form now or hereafter known for such purpose including,negative load threads, perpendicular threads flanks, buttress threads,etc.

Referring to FIGS. 1-7B, the polyaxial bone fixation element 10 ispreferably provided to the user in a kit including at least (1) boneanchors, (2) locking caps, and (3) pre-assembled collet/bodysubassemblies. The pre-assembled collet/body subassemblies arepreferably assembled by inserting the collet 150 into the axial bore 206formed in the body 200 through the upper opening 203 formed in the body200. The flexible arms 172 may flex inwardly as the collet 150 isinserted into the axial bore 206, if the greatest diameter of theflexible arms 172 is larger than the first diameter D1. Such aconfiguration generally results in the collet 150 being retained withinthe axial bore 206, even before the collet retention feature 50 isengaged. Once the collet 150 is positioned within the axial bore 206such that the flexible arms 172 are positioned proximate the enlargedportion 220, a force is applied to a distal end of the partialpassageway 216 formed in the body 200 so that a projection or dimple 216a is formed, which extends into the bore 206 of the body 200. Theprojection or dimple 216 a is positioned within the longitudinal groove175 formed in the collet 150 so that the collet 150 is free tovertically translate or float within the bore 206 with respect to thebody 200, but generally prevented from passing back up through the upperopening 203 formed in the body 200 and limited in its ability to rotaterelative to the body 200.

The kit is preferably shipped to the user for use in spinal surgery.During surgery, the surgeon preferably identifies a level of the spinewhere the surgery will take place, makes and incision to expose theselected area and implants the bone anchors 20 into the desiredvertebrae 700. The body/collet subassemblies are preferably popped-on tothe bone anchors 20 by urging the head 24 through the lower opening 205.Accordingly, the collet/body subassembly may be engaged with the headportion 24 of the bone anchor 20 in situ. Specifically, as the head 24moves into the lower opening 205, the collet 150 is urged toward andinto the loading position wherein the lower end of the longitudinalgrooves 175 contact the dimples 216 a. In the loading position, theouter curvate or spherical convex surface 151 of the collet 150 is ingeneral vertical alignment with the enlarged curvate or sphericalconcave surface of the enlarged portion 220 formed in the axial bore 206of the body 200. Alignment of the enlarged portion 220 with the collet150 enables the collet 150 to radially or outwardly expand so that thehead portion 24 of the bone anchor 20 can be received within the cavity165 formed in the collet 150.

Once the head 24 is positioned in the cavity 165, the head portion 24 ofthe bone anchor 20 and the collet 150 are both preferably constrainedwithin the body 200. The bone anchor 20 is preferably able topolyaxially rotate with respect to the collet 150 and the body 200 inthis configuration. The spinal rod 250 is inserted into therod-receiving channel 208 formed in the body 200 and onto the innersurface 161 of the seat 160. The spinal rod 250 is preferably positionedin facing engagement with the inner surface 161 of the seat 160. The setscrew 302 is preferably threaded into engagement with the threads 212formed in the body 200 to urge the spinal rod 250 and collet 150 towardthe lower end 204.

Rotation of the set screw 302 causes the bottom surface of the set screw300, preferably the saddle 310, to contact the top surface of the spinalrod 250, which in turn causes the spinal rod 250 and the collet 150 tomove downwards with respect to the body 200. Downward movement of thecollet 150 with respect to the body 200 causes line contact between theouter curvate or spherical convex surface 151 of the collet 150 and thefirst spherical undercut 218 a of the lower end portion 218. Additionaltightening of the set screw 300 and downward movement of the spinal rod250 and collet 150 results in an inwardly directed compressive force tothe flexible arms 172, causing the flexible arms 172 to pivot inwardlytoward the head portion 24 at their terminal ends 174 about the rootends 173. In a locking position, the flexible arms 172 engage the head24 of the bone anchor 20 securing the position of the bone anchor 20with respect to the collet 150 and the body 200. Specifically, theflexible arms 172 of the collet 150 radially compress against the headportion 24 of the bone anchor 20, which secures the position of the boneanchor 20 with respect to the body 200. The line contact between thecollet 150 and body 200 proximate the terminal ends 174 direct theradial inward force on the flexible arms 172 at a location preferablybelow the largest diameter of the head 24 to efficiently urge theterminal ends 174 beneath the curved outer surface of the head 24 in thelocked position. In addition, the spinal rod 250 is sandwiched betweenthe set screw 302 and the collet 150 in the locking position, therebysecuring the position of the spinal rod 250 with respect to the body 200and the bone anchor 20.

Referring to FIGS. 3 and 5A-6E, the collet 150 and body 200 may bepopped-off of the bone anchor 20, in situ, after the bone fixationelement 10 is engaged in the locked configuration. Specifically, the setscrew 300 may be removed from the body 200 and the spinal rod 250 may beextracted from the rod-receiving channel 208 and the seat 160. A tool(not shown) engages the ledges 184, 186 and the body 200 and applies aforce between the collet 150 and the body 200 to move the body 200downwardly toward the body anchor 20. The line contact between the body200 and the collet 150 is released and the collet 150 is urged into theloading position. In the loading position, the flexible arms 172 flexoutwardly within the enlarged portion 220 to permit popping-off of thebody 200 and collet 150 from the head 24 of the bone anchor 20. Thecollet 150 and body 200 may then be popped back onto the bone anchor 20.

Referring to FIGS. 4A, 4B and 8A-9B, the head portion 24 of the boneanchor 20 preferably includes a drive surface 30 in the form of aninternal drive recess or a first tool interface 32 and a second toolinterface 40. The second tool interface 40 preferably includes aplurality of threads 42 for threadably engaging the sleeve 600. Thesleeve 600 preferably accommodates a variety of surgical instruments nowor hereafter known including, but not limited to, the screw driver 500,a fluid delivery device such as the injection assembly 650, acompressor, a distractor, minimally invasive instrumentation, etc. Byincorporating the second tool interface 40 into the head portion 24 ofthe bone anchor 20, the surgical instruments are able to directly engagethe bone anchor 20, thus eliminating the need for the surgicalinstruments and/or the sleeve 600 to engage the body 200 or collet 150of the polyaxial bone fixation element 10. In addition, once the sleeve600 is engaged with the second tool interface 40, toggling between thescrew driver 500 or another instrument inserted through an internal borein the sleeve 600 and the bone anchor 20 is limited. In the preferredembodiment, the sleeve 600 is utilized to draw the screw driver 500 intothe first tool interface 32 to limit toggling between the screw driver500 and the bone anchor 20. The second tool interface 40 is describedherein as interacting or engaging the sleeve 600, but is not so limitedand may be configured to interact or engage with nearly any tool orinstrument that preferably is utilized to positively engage the head 24and apply forces to the bone anchor 20 for manipulating the bone anchor20, vertebra 700 mounted to the bone anchor 20 or any other structurethat is mounted to the bone anchor 20.

In order to facilitate implantation of the polyaxial bone fixationelements 10 and to perform, for example, one or more steps in a surgicalprocedure, it is advantageous to limit or remove “toggling” between thepolyaxial bone fixation elements 10 and any surgical instruments thatare utilized with the bone fixation elements 10. By incorporating thesecond tool interface 40 into the head portion 24 of the bone anchor 20,the sleeve 600 and, indirectly, the surgical instrument, for example,the screw driver 500, are directly connected to the bone anchor 20. Thesleeve 600 includes a threaded distal portion 602 for threadablyengaging the threads 42 of the second tool interface 40. In this manner,the sleeve 600 is directly coupled and secured to the bone anchor 20.Through this engagement, toggling is limited between the sleeve 600 andthe bone anchor 20. In addition, having a close tolerance between ininternal surface of the bore in the sleeve 600 and the screw driver 500or other instrument significantly limits toggling between the screwdriver 500 or other instrument and the bone anchor 20.

During the surgical procedure, the direct connection of the sleeve 600with the bone anchor 20 facilitates protection of the polyaxial lockingmechanism (the collet 150 and the body 200) of the polyaxial bonefixation element 10 and provides a more stable distraction, because theforces applied to the sleeve 600 are transferred directly to the boneanchor 20 via the second tool interface 40 and into the vertebra 700, asopposed to acting through these elements as well as the collet 150and/or the body 200, which may distort some of the forces and causetoggling. In addition, instruments, such as the screw driver 500 or theinjection assembly 650 may be securely positioned in engagement with thebone anchor 20 to drive the bone anchor 20 with the screw driver 500,inject bone cement or other fluid into and through the bone anchor 20 orotherwise conduct a procedure with the bone anchor 2, without operatingthrough the body 200 and/or collet 150.

The second tool interface 40 preferably does not obstruct access to thedrive recess 32, because the second tool interface 40 is preferablylocated above and radially outwardly relative to the drive recess 32.However, the second tool interface 40 is not limited to being locatedabove and radially outwardly relative to the drive recess or first toolinterface 32 and may be located below and radially inwardly relative tothe first tool interface 32, as long as tools or instruments are able toengage the first and second tool interfaces 32, 40 simultaneously.Specifically, the second tool interface 40 may be comprised of athreaded recess in the bone anchor 20 having a smaller diameter andlocated below the first tool interface 32. Moreover, as best shown inFIGS. 9A and 9B, the sleeve 600 preferably includes the longitudinalbore so that the screw driver 500, for example, may engage the driverecess 32 formed in the head portion 24 of the bone anchor 20. In thismanner, the sleeve 600 engages the bone anchor 20 via the second toolinterface 40 while the screw driver 500 simultaneously engages the driverecess 32.

Referring to FIGS. 8A and 8B, the sleeve 600 is preferably associatedwith a slip sleeve 800 that surrounds the sleeve 600 in a workingconfiguration. The sleeve 600 and the screw driver 500 are rotatablerelative to the slip sleeve 800 such that a surgeon may grasp the slipsleeve 800 while turning a handle 502 on a distal end of the screwdriver 500 to screw the bone anchor 20 into or out of the vertebra 700.When the surgeon rotates the handle 502, the sleeve 600, screw driver500 and bone anchor 20 each rotate relative to the slip sleeve 800.

In an anchor driving position (FIG. 8B), external threads 602 on thesleeve 600 are fully threaded into the threads 42 of the second toolinterface 40, thereby limiting any toggling between the sleeve 600 andbone anchor 20. The top 501 of the screw driver 500 is also fullyengaged with the drive recess 32 of the bone anchor 20 in the anchordriving position. When a surgeon has completed manipulating the boneanchor 20 with the screw driver 500, the screw driver 500 may be removedfrom the sleeve 600 and another instrument may be utilized with thesleeve 600 to gain access to the bone anchor 20.

Referring to FIGS. 9A and 9B, the injection assembly 650 may be utilizedwith the sleeve 600 to inject bone cement or other flowable materialsinto a cannulated bone anchor 20′. Once the bone anchor 20′ is mountedin the vertebra 700 and the sleeve 600 is mounted on the bone anchor20′, a cannula 652 of the injection assembly 650 is inserted into thesleeve 600. The cannula 652 is associated with a syringe 654 at aproximal end and includes a blunt tip 656 at a distal end. The cannula652 preferably includes an engagement mechanism 656 proximate theproximal end to engage a proximal end of the sleeve 600 to secure thecannula 652 to the sleeve 600. In addition, the engagement mechanism 656preferably urges the blunt tip 656 of the cannula 652 into engagementwith a shelf 32 a at the bottom of the first tool interface 32 such thata flow channel 34 of the cannula 652 is in communication with a flowchannel 652 a of the cannula and a seal is created between the blunt tip656 and the shelf 32 a. In the preferred embodiment, the engagementmechanism 656 is comprised of a threaded joint that may be tightened tosecure the cannula 652 relative to the sleeve 600 and securely seal theblunt tip 656 to the shelf 32 a, thereby generally preventing leakage ofbone cement or other flowable fluid into the first tool interface 32.Bone cement is preferably injected into the flow channel 652 a with theinjection assembly 650 and into the vertebra 700 to securely mount thebone anchor 20′ to the vertebra 700, particularly in generally weak,brittle and/or osteoporotic bone. The bone anchor 20′ may also befenestrated to inject bone cement toward the sides of the bone anchor20′ or to generally directionally dispense the bone cement or otherfluid. Further, the bone anchor 20′ may be utilized to extract materialfrom the vertebra 700 or other bone that the bone anchor 20′ is engagedwith by drawing the material into the flow channel 652 a, by, forexample, creating a vacuum in the flow channel 652 a. For example, thebone anchor 20′ may be utilized to extract bone marrow from the bone.Further, the bone anchor 20′ may be utilized to aid in bone graftextension, as would be apparent to one having ordinary skill in the art.

It should be understood that while the bone anchor 20 is being describedherein as preferably including the second tool interface 40, the secondtool interface 40 is optional. Furthermore, it should be understood thatthe bone anchor 20 including the second tool interface 40 may be used inany other type of bone screw application such as, for example, long bonefixation, fracture fixation, or in connection with securing a boneplate, vertebral spacer, dental implant, etc.

The polyaxial bone fixation element 10 including the bone anchor 20, thecollet 150, the body 200 and the locking cap 300 may be made from anybiocompatible material now or hereafter known including, but not limitedto, metals such as, for example, titanium, titanium alloys, stainlesssteel, Nitinol, etc.

As will be appreciated by those skilled in the art, any or all of thecomponents described herein may be provided in sets or kits so that thesurgeon may select various combinations of components to perform afixation procedure and create a fixation system which is configuredspecifically for the particular needs/anatomy of a patient. It should benoted that one or more of each component may be provided in a kit orset. In some kits or sets, the same device may be provided in differentshapes and/or sizes.

While the foregoing description and drawings represent the preferredembodiment of the present invention, it will be understood that variousadditions, modifications, combinations and/or substitutions may be madetherein without departing from the spirit and scope of the presentinvention as defined in the accompanying claims. In particular, it willbe clear to those skilled in the art that the present invention may beembodied in other specific forms, structures, arrangements, proportions,and with other elements, materials, and components, without departingfrom the spirit or essential characteristics thereof. One skilled in theart will appreciate that the invention may be used with manymodifications of structure, arrangement, proportions, materials, andcomponents and otherwise, used in the practice of the invention, whichare particularly adapted to specific environments and operativerequirements without departing from the principles of the presentinvention. In addition, features described herein may be used singularlyor in combination with other features. The presently disclosedembodiments are, therefore, to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims and not limited to the foregoingdescription.

It will be appreciated by those skilled in the art that changes could bemade to the embodiments described above without departing from the broadinventive concept thereof. It is understood, therefore, that thisinvention is not limited to the particular embodiments disclosed, but itis intended to cover modifications within the spirit and scope of thepresent invention, as defined by the appended claims.

What is claimed:
 1. A polyaxial bone fixation element for coupling alongitudinal spinal rod to a vertebra, the bone fixation elementcomprising: a bone anchor including an enlarged, curvate head portion,wherein the head portion includes a first tool interface for engaging afirst surgical instrument and a second tool interface for engaging asecond surgical instrument, the first and second tool interfacespositioned in a head interface cavity; a body for interconnecting thebone anchor and the longitudinal spinal rod, the body having alongitudinal axis, an upper end with an upper opening, a lower end witha lower opening, a bore extending substantially between the upperopening and the lower opening, the bore having a first diameter D₁, arod-receiving channel for receiving the spinal rod located at the upperend, the rod-receiving channel having a channel axis that is orientedsubstantially perpendicular to the longitudinal axis, the body furtherincluding a lower edge portion adjacent the lower opening, the loweredge portion defining a second diameter D₂ smaller than the firstdiameter D₁; a collet including a first end, a second end and one ormore slots extending from the second end, the slots defining a pluralityof flexible arms, the collet movably positioned within the bore of thebody; and a locking cap removably engageable with the body, the lockingcap being movable from an unlocked position to a locked position whereinmovement of the locking cap from the unlocked position to the lockedposition urges the rod against the collet and the flexible arms againstthe lower edge portion to secure a position of the bone anchor relativeto the body.